LAWS OF ELECTROLYTIC DISSOCIATION 35 



liberation by the HCl from the bicarbonate of the true and strong 

 H2CO3, which then gradually decomposes into CO2 and H2O. This 

 experiment is made possible by the relatively slight velocity of the 

 reaction 



H2COS-* H2O + CO2 



This case of an "apparent dissociation constant" is by no means 

 limited to carbonic acid alone. It must be remembered that it is 

 in reality quite impossible to distinguish in a given case a true 

 from an apparent dissociation constant. This point is so impor- 

 tant that it will be amplified by some examples. Glucose is an ex- 

 traordinarily weak acid. In attempting to picture the chemical 

 constitution of glucose and of its ions we face the possibility of repre- 

 senting the constitution of this sugar in a number of various ways. 

 We may regard it, according to E. Fischer's original structural 

 formula, as an aldehyde, or, according to ToUens, as a cychc com- 

 pound with its ring closed by means of one 0-atom. The latter 

 may exist in two stereo-isomeric forms as the a- and /3-glucose. 

 Recently even more isomers of the cyclic kind were found. All 

 of these forms are actual and coexist in a state of equilibrium. 

 Furthermore, another configuration is possible, the enolic form of 

 Wahl and Neuberg which is derived from Fischer's aldehyde form. 

 The formulas below show this derivation: I is the aldehyde which 

 in II forms a hydrate and, as is shown in II, the hydrate splits off a 

 molecule of H2O again to form the enol form shown in III. 



/OH 

 H— C=0 H— C< H— C— OH 



OH— C— H OH— C— 



OH 

 H 



OH— C 



c c c 



i i i 



I II III 



Aldehyde Aldehyde Enol 



hydrate 



This enolic form is common to glucose, mannose and fructose. 

 Thus, when this enol adds a molecule of H2O and passes to the 

 aldehyde again, the resulting sugar of this process may be either 

 glucose, mannose or fructose, and in fact the velocity of this process 



